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What it takes to get a zero backlash gearbox?

In a motion system, a gearhead, or a speed reducer, is a component of critical importance to deliver desired torque quantities. In fact, its primary function is to multiply moment of force by creating a mechanical advantage. It does the job all right, but for backlash causing motion losses, which impairs positioning accuracy and downgrades overall system performance. One way to avoid the undesirable effects is to get a zero backlash gearbox.

The definition

To understand better what the backlash is, it is essential to have a clear idea of the gearhead mechanics. Structurally, a gearbox is an arrangement of mechanical components, such as pinions, bearings, pulleys, wheels, etc. Exact combinations vary, depending on specific reducer type. What’s common for all combinations—they are intended to transmit power from the motor output towards the load so as to reduce speed and increase torque in a safe and consistent manner.

Backlash, also lash or play, is the gap between the tail edge of the tooth transmitting power from the input and the leading edge of the immediately following one. The gap is essential for gears to mesh with each other without getting stuck and to provide lubrication within the casing. On the downside, the mechanical play is associated with significant motion losses, preventing a motor from reaching its optimal performance. First of all, the losses impact negatively efficiency and precision.

Incorrect tolerances, bearing misalignment, and manufacturing inconsistencies tend to increase backlash. Though getting rid of it entirely is neither possible nor reasonable, minimizing it to almost zero values can help to avoid the above described negative effects. So, how do we do it?

How to get a zero backlash gearbox

There are several ways in which it is possible to provide a gearbox with zero backlash, thus enabling high precision motion control.

Modifying construction

There is a wide range of modification techniques, but the most commonly used are as follows:

Decreasing the width between centers. Smaller between-center distances are achieved either by securing a gearwheel in place with preset spacing or by inserting a spring. Rigid bolted assembly is typical of bidirectional gearboxes of the bevel, spur, worm or helical type in heavy-duty applications. Spring loading is a better choice to keep lash at acceptable values in low-torque solution. Mind that the locked-in-place arrangement requires in-service trimming since teeth tend to wear with time.

Tapering.  According to the technique, wheels, such as helical or spur, have their teeth tapered. The play in a gearbox is trimmed during assembly by axial displacement of the wheels relative to each other.

Inserting a plastic filler. The filler, usually a piece of elastic material, is passed through a mating gearwheel at the center, protruding beyond the teeth profile to fill the lash. However, as the filler material wears over the service life, the lash is back to its initial value.

Splitting. The split arrangement implies mounting two halves of a gearwheel onto a shaft side by side with a spring in between. One half is secured in place, while the other is forced to turn slightly by the spring. The technique is commonly used in systems where speeds and loads are low.

Preloading. To eliminate the clearance between the interfacing teeth, a torsion spring or a load is coupled to the last driven gearwheel. The technique proves especially efficient in multi-stage applications, where play is a cumulative magnitude. Those are typically low-torque engines rotating in one direction only. The biggest issues with the configuration is that the preloading impairs free spinning. To eliminate the problem, it is advisable to replace them with an auxiliary motor.

Dual-path configuration. In the configuration, two identical gearing sets are mounted in a parallel arrangement and gyrate in opposite directions. Additionally, the arrangement is preloaded: a motor shaft is inserted together with a pinion into the gearhead. The downsides are doubled quantity of components and extended assembly time.

Utilizing inherently precise gear sets

Building an ultra-precise gearbox requires taking measures to avoid workmanship defects and ensuring close-tolerance alignment of components in a mechanism. Possible measures include custom machining techniques and enhanced dimensional control prior to and during assembly. Manufacturers also introduce safe handling and packaging practices to exclude post-production damages, such as chips, or dirt contamination. In addition, speed reducers with high precision are typically produced in small batching, which enables thorough quality testing.

The efforts naturally pay off, enabling to cut lash down to 2 degrees or even less—the kind of accuracy required for instrumentation, robots, or machine tools.

Employing motion solutions of special design

Strain-wave, as well as cycloidal and epicyclic designs incorporate no conventional racks, gears, or pinions, thus enabling to obtain a zero backlash gearbox. The speed reducers are expensive, for which reason their use is limited to automation solutions where efficiency and high precision are critical to the extent the cost ceases to be an issue.

In cycloidal mechanisms, torque transfer is via preloaded balls, rollers, or pins. The mechanisms feature 95% efficiency, generate low noise, are capable of sustaining shocks and resistant to vibration. Though lash-free, the devices require readjustment throughout life-in-service.

Enabling to reduce backlash to 0.5 to 5 arcmin, epicyclic (planetary) mechanisms comprise a central (sun) and outer (planet) wheels. The planet wheels are attached to a spinning carrier element and rotate about the sun element. Their biggest advantages are high torsional stiffness and low inertia.

A strain-wave (harmonic) gearhead, like those used with RDrive servo motors by Rozum Robotics, is a metal structure of unique design relying on metal elasticity properties. The construction is easy to assemble—it comprises three elements only: a wave generator, a flexspline, and a circular spline. The simplicity makes the devices exceptionally reliable, while enabling reconfiguration to fit application-specific requirements.

With zero backlash, the reducer boasts minimized positioning inaccuracies and efficiency losses due to gearing and excellent repeatability. Though compact in size, the device has high torque capacity. In RDrive series models, the gearbox is embedded into the servo housing. With the ratio of 1:100, it enables multiplying the output of brushless AC drives by 100 times. Quiet operation and minimal vibration are additional bonuses.